1. Field of the Invention
This invention relates to the recovery of oil from a subterranean, viscous oil-containing formation and more particularly to an improved thermal recovery method involving the injection of a predetermined amount of high temperature steam into the formation prior to a steam drive process to improve displacement efficiency and enhance oil recovery.
2. Background of the Invention
In the recovery of oil from oil-containing formations, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the formation. Thus, a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean formations. One of the most widely used supplemental recovery techniques is the steam drive recovery process, which involves the injection of steam into the formation by means of an injection well and oil is recovered from the formation from a spaced-apart production well. As the steam moves through the formation, it not only serves as a driving force to cause the oil to flow into the producing well, but it gives up its heat to the oil thereby lowering the viscosity of the oil over a substantial portion of the formation, and enhancing the recovery of the oil.
In a steam drive recovery process, gravity overrides and viscous figuring are often experienced. Gravity overrides is associated with the fact that steam, being of lower density than other fluids present in the permeable formation, migrates to the upper portion of the permeable formation and channels across the top of the oil formation to the remotely located production well. Thus, steam override results in very little oil being recovered from the lower portions of the formation. Viscous fingering occurs when the displacing fluid comprising steam or condensed steam tends to finger through the oil phase towards the producing well, destroying piston-like displacement and resulting in premature breaksthrough of the displacing fluid resulting in much of the oil being bypassed. Viscous fingering is dominantly caused by the large differences in oil and water viscosities resulting in a high water/oil mobility ratio which has an adverse effect on areal sweep efficiency or displacement efficiency of the displacing fluid. Most favorable mobility ratios and greater displacement efficiencies are therefore obtained when there is little difference between the viscosity of the displacement fluid and the oil.
U.S. Pat. No. 3,439,741 to Parker discloses a steam drive oil production process wherein slugs of heavy crude oil are periodically injected into the steam injection well with either continuous or intermittent injection of steam to upgrade the injected crude oil by visbreaking thereby reducing fingering and improving the production of oil. The temperature of the injected steam is at least 550.degree. F., and is usually in the range of 550.degree. to 650.degree. F.
This invention reduces viscous fingering in a steam drive process by forming in-situ a zone of visbroken oil in the formation located between the in-place oil and the injected steam that provides a more favorable mobility ratio between the phases thereby improving the sweep efficiency of the steam drive.